Why did America enter the war in Europe when it did?

In the two most recent, most dreadful, wars in Europe - World War I and World War II - America entered 'late'.

In World War I, which began on 28 July, 1914 - the Americans entered the war when their Expeditionary Forces arrived on 3 May 1917.

The reason given for America joining was the Germans attacks on American neutrality by way of attacks on international shipping.

In World War II, which started on 1 September, 1939 - the Americans entered the war in Europe (after Pearl Harbour, the Germans joined Japan in declaring war on America). The first American forces arrived in Europe on 26 January, 1942.